My group's currently playing what we call a "roulette" campaign where we trade off GMs every adventure and its been pretty fun so far. All but one of us has GMed for this group before. As GMs none of us are strangers to playing the GM card and cheating a little bit to make the game more fun for everyone, but we never blatantly state that we are cheating the system and if a GM got a rule wrong at a player's expense we generally try to reach a compromise.
The other day our party was ambushed while we were asleep at camp (apparently NPCs make bad sentries) and I rolled a flat 3 (no ranks or WIS) on my Perception check to wake up from the sound of combat. The GM said that I had failed the roll and missed the surprise round and was automatically last on initiative. Okay, that's fine, but I didn't think a 3 was actually a failure.
Looking up the rules I quickly found that I had actually succeeded at the roll: Perception check to hear the sound of battle is a DC: -10. You get -10 to Perception checks made to hear things while asleep, therefore my roll was a -7 and beat the -10 DC. When I brought this up before the surprise round was over my GM simply said, "Nope, GMs can cheat you are asleep."
I was kind of peeved especially because he let everyone have there armor on despite no-one having Endurance, so I was really the only one being screwed over. It wasn't a huge deal as I am the healer anyway but I still have a thirst for revenge.
When it's my turn at bat I kind of want to get him back and this is where you guys come in: I need a prank to pull when I'm GMing the campaign that won't do any kind of permanent damage to his character and that blatantly breaks the rules in a situation where game mechanics and rules are important (combat is the most obvious). Something funny and tongue-in-cheek. Not something wholly malicious or detrimental to the other players.